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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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GLACIATION OF LOWER COPPER RIVER                      457
cipitous 1800 foot cliff east of the terminus of Allen Glacier, where the lower 1000 feet are nearly vertical, (2) the western canyon wall at Abercrombie Rapids, and (8) the western side of the isolated 600 foot rock hill at the mouth of La Gorce Valley. Part of this rock, facing Allen Glacier, has vertical and, at one place, actually overhanging cliffs. In each case the absence of striation, which is unquestionable at the localities inspected, and seems to be so for the inaccessible higher portions of the same cliffs examined with field glasses, is explained by glacial plucking. The absence of striation on two of these cliffs, however, opposite marginal channels of the Copper River, suggests the complication of stream erosion. Below these non-striated cliffs there is usually an absolute absence of talus, showing that the rough surfaces were produced by an erosive agent which worked in association with a transporting agency. This eliminates post-glacial weathering or avalanching.
Talus slopes mantle certain of the canyon walls in the section between Tasnuna River and Childs Glacier. These are especially well developed at Abercrombie Rapids, where the railway engineers have cut off the noses of several steep talus cones which are 1000 or 2000 feet high. This has modified them in a way to invite future rock avalanches.1 These talus cones, some of which extend above the upper limit of glaciation, and which are now entirely covered with vegetation are, therefore, no longer growing, from which it may be inferred that post-glacial, high-altitude weathering is far less effectual than the super-glacial weathering of the time of maximum glaciation.
Some of the tributaries of Copper River are in hanging valleys (PI. CLXXIX). The smaller tributaries have discordances of several hundred feet, the stream at Whiting Falls north of Heney Glacier, for example, hanging 300 to 400 feet, another (Map 9) just southwest of Allen Glacier hanging 550 feet, and a third just north of Abercrombie Rapids hanging about 400 feet. This last hanging valley is at Camp 55 and a milky stream from a small glacier descends its lip, but the gorge of the stream just north of it, which has no glacier, is not a hanging valley. Shields Glacier is in a hanging valley, but it is not certain whether the large tributary streams, like those of La Gorce Glacier and Wernicke River are in hanging valleys or not. If so, their discordance is hidden by the gravel filling of the Copper River canyon. Bremner River may even be discordant in relation to the Tasnuna and main Copper Rivers, for there are low, roches moutonn£e hills in its mouth.
The floor of the middle section of the Copper River canyon is absolutely devoid of rock ledges, excepting in rare hills like the large one at the mouth of La Gorce Valley and small rock islands close to the valley walls, as in a case just south of Wernicke River. The rock bottom is thought to be some distance below the present river level, perhaps partly accounting for the greater width here than in the section north of Tasnuna River. The valley floor is mantled with glacial outwash, which covers about fifty square miles between Allen Glacier and Tasnuna River. The present grade of Copper River for the whole middle section of the canyon averages less than three and one-half feet to the mile, or about hah* that in the section between Tasnuna River and Chitina. As this three and one-half foot average includes the Childs,1 Abercrombie, and Baird Rapids, it is apparent that the grade elsewhere is not over 2 or 8 feet to the mile. The Allen (PL CLXXXI, B) and Miles Glaciers, acting as a dam and checking
»Some of these predicted avalanches took place during the summer of 1912, blocking the railway for several weeks.